This application is an continuation of Application No. 12/683681, filed on Jan. 7, 2010, now Pat. No. 8,629,432.
1. Field of the Invention
The present invention relates to a semiconductor device including a circuit which includes a thin film transistor (hereinafter, referred to as TFT) and a manufacturing method thereof. For example, the present invention relates to an electronic appliance mounted with an electro-optic device typified by a liquid crystal display panel or a light-emitting display device including an organic light-emitting element as a component.
In this specification, a semiconductor device refers to all types of devices which can function by using semiconductor characteristics. An electro-optical device, a semiconductor circuit, and an electronic device are all included in the category of the semiconductor device.
2. Description of the Related Art
There are a variety of kinds of metal oxides intended for many uses. Indium oxide is a well-known material and is used as a transparent electrode material necessary for a liquid crystal display or the like.
Some metal oxides have semiconductor characteristics. The metal oxides having semiconductor characteristics are one kind of compound semiconductor. The compound semiconductor is a semiconductor formed by two or more kinds of atoms bonded together. In general, metal oxides are insulators; however, it is known that metal oxides are semiconductors depending on the combination of elements included in the metal oxides.
For example, among metal oxides, tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like are known as the metal oxides having semiconductor characteristics. The metal oxide as above has been used for a transparent semiconductor layer serving as a channel formation region in a thin film transistor (as disclosed in Patent Documents 1 to 4 and Non-Patent Document 1).
Examples of metal oxides include not only an oxide of a single metal element but also an oxide of a plurality of metal elements. For example, InGaO3(ZnO)m (m is a natural number) which is a homologous compound is a known material (Non-Patent Documents 2 to 4).
It has been confirmed that such an In—Ga—Zn based oxide as above is applicable to a channel layer of a thin film transistor (Patent Document 5 and Non-Patent Documents 5 and 6).
Further, attention has been drawn to a technique for manufacturing a thin film transistor using an oxide semiconductor and applying to an electronic device or an optical device. For example, Patent Document 6 and Patent Document 7 disclose a technique by which a thin film transistor is manufactured using zinc oxide or an In—Ga—Zn—O based oxide semiconductor as an oxide semiconductor film and such a transistor is used as a switching element or the like of an image display device.